High-performance emitter for thermoelectronic diodes

ABSTRACT

A high-performance emitter for thermoelectronic diodes, and a method for manufacturing said emitter. The emitter comprises a fissile fuel member 2 coated with at least one refractory metal emitting layer 7 deposited on the periphery thereof, and is characterized in that said member is a solved bar provided with radial cuts, or grooves 3, made by machining said member and adapted to act as reinforcing hoops once said layer has been deposited. The method consists in depositing layers in an atmosphere constituted by hydrogen and gaseous molybdenum and tungsten halogenides (or habides) at a temperature of at least 2,190*F (1,200*C) under a pressure of at most 0.04 in (1mm) of mercury.

United States Patent [1 1 Durand et al.

4 1 May 13, 1975 HIGH-PERFORMANCE EMITTER FOR THERMOELECTRONIC DIODES Inventors: Jean-Paul Durand, Par is; Jacques Gillardeau, Longjumeau; Robert Faron, Nyon, all of France Commissariat a IEnergie Atomique, Paris, France Filed: May 25, 1973 Appl. N0.: 363,867

Assignee:

Foreign Application Priority Data June 2, 1972 France 72.19985 US. Cl. 313/346; 29/2514; 29/2517; 313/54 Int. Cl H0lj l/14; H01j 19/06 Field of Search 313/346, 345, 54, 346 DC; 29/2514, 25.17

References Cited UNITED STATES PATENTS 12/1955 Nelson 313/54 X l/1959 Van Tol 313/346 R 2/1964 Haas 313/54 X 1/1972 Weis 313/346 R Primary Examiner-James W. Lawrence Assistant ExaminerSaxfield Chatmon, Jr.

Attorney, Agent, or Firm-Cameron, Kerkam, Sutton, Stowell & Stowell [57] ABSTRACT A high-performance emitter for thermoelectronic diodes, and a method for manufacturing said emitter.

The method consists in depositing layers in an atmosphere constituted by hydrogen and gaseous molybdenum and tungsten halogenides (or habides) at a temperature of at least 2,190F (1,200C) under a pressure of at most 0.04 in (1mm) of mercury.

11 Claims, 2 Drawing Figures HIGH-PERFORMANCE EMITTER FOR THERMOELECTRONIC DIODES The present invention relates to the manufacture of high-performance emitters intended for thermoelectronic diodes.

Emitters of the prior art are constituted by a solid bar or by stacked fissile material elements suitably sheathed with duly machined refractory material.

The present invention aims at taking advantage of the specific properties of metal deposits obtained by transfer in the vapour phase and chemical decomposition.

By means of such deposits, it is possible to obtain layers of even thickness on parts having various configurations, by combining the close contact properties be tween a layer so deposited and the substrate. Such deposits also permit to achieve mechanical and emissive features which can be controlled according to the conditions under which the material is being deposited.

The present invention relates to an emitter per se and to a method for manufacturing same.

More specifically, the present invention relates to a thermoelectronic emitter comprising a fissile fuel member coated with at least one refractory metal emitting layer deposited on the periphery of said member, wherein said fissile fuel member is a solid bar provided with radial cuts, or grooves, made by machining said member and adapted to act as reinforcing hoops once said layer has been deposited.

According to another featured the invention, said fissile fuel member consists of a stack of fissile fuel pellets and metal cross-pieces, said stack being sheathed by said refractory metal emitting layer.

The hoops or sheathes of refractory metal act as mechanical means for reinforcing the fissile fuel member and rendering it able to withstand deformations resulting from the pressure built up by the fuel or by the fission gases in the course of irradiation.

According to a further feature, the emitting layer consists of a tungsten layer which acts both as a mechanical reinforcing means and as an emitter.

The emitter performance can be improved by forming the emitting layer as a tungsten reinforcing deposit coated with a film of tungsten, the grains of which are exclusively defined by the (110) plane of tungsten.

According to a still further feature, the adhesive power of the layers is remarkably improved by directly applying a molybden layer onto the fissile fuel member, before depositing the tungsten emitting layer.

With a view to substantially preventing any diffusion between molybdenum and tungsten, it is advisable to deposit a molybden-tungsten alloy of appropriately selected composition between the above two layers.

The method for manufacturing the emitter according to the present invention distinguishes from the prior art in that the deposition of said layers is carried out in an atmosphere constituted by hydrogen and gaseous molybdenum and tungsten halogemides (or halides) at a temperature of at least 2,19()F 1,200C) under a pressure of at most 0.04 in lmm) of mercury.

The growth anisotropy of the tungsten layer is remark-ably increased at temperatures above 2,730F l,50()C) under the same pressure conditions over the whole surface of the deposit.

The stability of tungsten metal atoms can be improved by modifying their balance profile through the appropriate addition to the method gases of a third gas, e.g., chlorine or oxygen.

In this way, the emitter is coated with a tungsten film having grains exclusively defined by the 1 l0) plane of tungsten.

The invention will be more fully described hereunder, with reference to the accompanying drawing, in which:

FIG. 1 is an axial cross-section of an emitter according to the invention; and

FIG. 2 is an axial cross-section of a second embodiment.

Emitter 1 according to the invention, such as shown in FIG. 1, is constituted by a fissile fuel member 2 of cylindrical shape, made for instance of enriched uranium oxide in metal form. Cuts or grooves 3, made by machining member 2, are evenly distributed over the whole periphery 4 thereof.

In the embodiment shown in FIG. 2, fissile fuel member 2 consists of a stack of fuel pellets 5 wedged between cross-pieces 6. Such a stack may also consist of pellets laid on a mandrel (not shown).

In FIG. 1, cuts or grooves 3 are filled with a refractory metal 7, for instance molybdenum or tungsten.

In FIG. 2, layer 7 of refractory metal acts as a reinforcing means and as a sheath for closely binding pellets 5 and cross-pieces 6. In both cases, such reinforcing means are necessary for withstanding deformations resulting from the pressure built up by the fuel or by the gases in the course of irradiation.

Emitting layer 7 is constituted by an even tungsten deposit acting as a reinforcing means and by a tungsten deposit 8, the grains of which are exclusively defined by the 1 l0) plane of tungsten. Such deposits can be made on a layer of molybdenum directly applied on the fissile fuel member 2.

Thus is obtained a reinforced emitter containing the nuclear fuel and which is usually adopted for the manufacture of thermoelectronic diodes.

The method for manufacturing the above two embodiments of the emitter according to the invention consists in depositing at least one metal layer obtained through the chemical decomposition of a volatile compound of the metal.

In the examples corresponding to FIGS. 1 and 2, a first layer of a refractory metal such as tungsten or melybdenum is sprayed by vaporization over the outer surface 4 of fuel member 2.

In the case of emitter 1 shown in FIG. I, masks may be applied against member 2, in order that the refractory metal be deposited only in grooves 3, according to the desired thickness. In the case of member 2 such as shown in FIG. 2, the deposit is made over the whole outer surface of said member, with a view to forming a sheath which will connect pellets 5 and cross-pieces 6.

Once the reinforcing layer has been laid, an emitting layer (not shown) is evenly distributed over the whole surface 4 of emitter 1, either in direct contact therewith, or on an intermediate layer of tungstenmolybdenum alloy previously deposited under the same conditions.

All that tungsten layer 7 is improved by means of a further and ultimate deposit 8 of tungsten, the grains of which are in the direction of plane 1 10).

The manufacturing steps are carried out in an atmosphere made of hydrogen and of gazeous molybdenum and tungsten halogenides, in particular hexafluorides WF6 and M 1 Such a reduction through hydrogen is achieved at temperatures above 2,1 90F (1,200C) and at low pressure.

The sum-total of the hydrogen pressure and the gaseous fluoride pressure is kept under 0.04 in. (1mm) of mercury. Under such conditions, the velocity at which metal atoms are deposited onto the substrate is lower than the velocity at which such metal atoms are set in the cristalline surface lattice, with the result that the deposits thus formed grow in epitaxic relation with respect to the substrate. This important feature provides the deposit so obtained with a noticeable adhesive power, since the cristalline lattice of the deposited metal constitutes a perfect extension of the cristalline lattice of the underlying metal.

Quite naturally, under such specific growth conditions, the interface between the substrate and the deposit is always excellent. In addition, it is to be noted that, since the epitaxic growth develops layer by layer, it leads necessarily to the formation of a perfectly compact deposit in which no porosity whatever can be detected.

Moreover, the anisotropy of the growth of tungsten is caused to increase considerably at temperatures above 2,730F (1,500C), until the cristalline plane of tungsten be thermo-dynamically stable over the whole surface of the tungsten deposit.

Indeed, under such temperature conditions and at low pressure, the surface freedom of motion of the metal tungsten atoms on the deposit which is in process of growing is such that these atoms represent by preference the balance profile which corresponds to the formation of high density planes. Provided an appropriate gas is added to the method gas, the balance profile can be changed. More specifically, by adding chlorine or oxygen, it is possible to develop in the emitter a tungsten film, all the grains of which are exclusively defined superficially by the (110) plane of tungstene, which plane is the most stable and dense, and, accordingly, the best one for promoting a thermoelectronic emmission.

The emitter according to the invention is manufactured, for example, either from a solid bar of fissile fuel (forming the substrate), in which cuts are made, or from fuel pellets wedged between cross-pieces.

According to a preferred embodiment, the various layers of refractory metals are obtained by depositing on the whole substrate surface, the following layers in succession;

a layer of molybdenum for obtaining the means for mechanically reinforcing the nuclear fuel.

an even layer of molybdenum.

an even layer of molybdenum-tungsten,

an even layer of tungsten, and

a thin layer of tungsten, the grains of which are, superficially, in the direction of the 1 l0) plane of tungsten.

What is claimed is:

l. A high-performance emitter for thermoelectronic diodes comprising a fissile fuel member, at least one refractory metal emitting layer deposited on the periphery of said member, said fissile fuel member being a solid bar, radial cuts, in said bar said cuts forming reinforcing hoops when said layer has been deposited.

2. A high-performance emitter according to claim 1,

wherein said fissile fuel member consists of a stack of fissile fuel pellets and metal cross-pieces, and a sheath of said refractory metal emitting layer around said stack.

3. A high performance emitter according to claim 1 said layer including a reinforcing layer of tungsten.

4. A high-performance emitter according to calim 1 said layer including a reinforcing layer of molybdenum.

5. A high-performance emitter according to claim 1 said layer including an emitting layer of tungsten, the grains of which are exclusively defined by the plane of tungsten.

6. A high-performance emitter comprising, in combination, a layer of molybden mechanically reinforcing a nuclear fuel, an even layer of molybdenum, an even layer of molybdenum-tungsten, an even layer of tungsten, and a thin layer of tungsten, the grains of which are, superficially, in the direction of the 1 l0) plane of tungsten.

7. A method for manufacturing a high-performance emitter for thermoelectronic diodes having a fissile fuel member coated with at least one refractory metal emitting layer on the periphery of said member with radial cuts in said member forming reinforcing hoops when said layer has been deposited, the step of deposition of said layers being carried out in an atmosphere of hydrogen and gaseous molybdenum and tungsten halogenides (or habides) at a temperature of at least 2,190F (1,200C) under a pressure of at most 0.04 in (1mm) of mercury.

8. A method according to claim 7, the deposition step being carried out in an atmosphere of gaseous halogenides of the metal which is deposited containing molybdenum and tungsten.

9. A method according to claim 7 the deposition of the tungsten layer with strictly defined grains being carried at a temperature above 2,730F 1,500C) to provide an epitaxied of tungsten at a pressure of at most 0.04 in (1mm) of Hg.

10. A method according to claim 7 including the step of adding compounds of the chlorine group to the atmosphere to promote the formation of the balance profile corresponding to the (l 10) plane of tungsten.

l l. A method according to claim 7 including the step of adding compounds of the oxygen group to the atmosphere to promote the formation of the balance profile corresponding to the (l 10) plane of tungsten. 

1. A HIGH-PERFORMANCE EMITTER FOR THERMOELECTRONIC DIODES COMPRISING A FISSLE FUEL MEMBER, AT LEAST ONE REFRACTORY METAL EMITTING LAYER DEPOSITED ON THE PERIPHERY OF SAID MEMBER, SAID FISSILE FUEL MEMBER BEING A SOLID BAR, RADICAL CUTS, IN SAID BAR
 2. A high-performance emitter according to claim 1, wherein said fissile fuel member consists of a stack of fissile fuel pellets and metal cross-pieces, and a sheath of said refractory metal emitting layer around said stack.
 3. A high performance emitter according to claim 1 said layer including a reinforcing layer of tungsten.
 4. A high-performance emitter according to calim 1 said layer including a reinforcing layer of molybdenum.
 5. A high-performance emitter according to claim 1 said layer including an emitting layer of tungsten, the grains of which are exclusively defined by the (110) plane of tungsten.
 6. A high-performance emitter comprising, in combination, a layer of molybden mechanically reinforcing a nuclear fuel, an even layer of molybdenum, an even layer of molybdenum-tungsTen, an even layer of tungsten, and a thin layer of tungsten, the grains of which are, superficially, in the direction of the (110) plane of tungsten.
 7. A method for manufacturing a high-performance emitter for thermoelectronic diodes having a fissile fuel member coated with at least one refractory metal emitting layer on the periphery of said member with radial cuts in said member forming reinforcing hoops when said layer has been deposited, the step of deposition of said layers being carried out in an atmosphere of hydrogen and gaseous molybdenum and tungsten halogenides (or habides) at a temperature of at least 2,190*F (1,200*C) under a pressure of at most 0.04 in (1mm) of mercury.
 8. A method according to claim 7, the deposition step being carried out in an atmosphere of gaseous halogenides of the metal which is deposited containing molybdenum and tungsten.
 9. A method according to claim 7 the deposition of the tungsten layer with strictly defined grains being carried at a temperature above 2,730*F (1,500*C) to provide an epitaxied of tungsten at a pressure of at most 0.04 in (1mm) of Hg.
 10. A method according to claim 7 including the step of adding compounds of the chlorine group to the atmosphere to promote the formation of the balance profile corresponding to the (110) plane of tungsten.
 11. A method according to claim 7 including the step of adding compounds of the oxygen group to the atmosphere to promote the formation of the balance profile corresponding to the (110) plane of tungsten. 